Frequency selective L1 repeaters controlled by an eNodeB are being considered for inclusion in the Long term Evolution (LTE)-Advanced concept currently being developed by 3GPP. A frequency selective repeater can be controlled by an eNodeB to only repeat resources that are in use by user equipment (UE) scheduled by that eNodeB. Furthermore, a UE with a strong radio link to the serving eNodeB does not need support from the repeater. Hence, resources used to communicate to and from that UE should be turned off by the repeater so that unnecessary interference is not forwarded by the repeater.
In order for frequency selective repeaters to work efficiently, the eNodeB needs to be able to associate each UE with one or more serving repeaters, as illustrated in FIG. 1. For example, eNodeB 115 may control repeaters 110-1 and 110-2 and serve UEs 105-1, 105-2, and 105-3. The UEs may be associated with one or more repeaters. Thus, one problem that needs to be solved in order for frequency selective repeaters to work as intended is that eNodeB 115 needs a method to obtain a repeater association for each UE. For example, either the UE is connected directly to the serving eNodeB or via one or more controlled repeaters.
There are existing techniques to associate a repeater with a serving eNodeB. For example, as illustrated in FIG. 2, repeater 110-1 may transmit a reference signal to UE 105-2. UE 105-2 may perform measurements based on a transmitted reference signal. UE 105-2 may report the measurements to eNodeB 115. The feedback report from UE 105-2 must be based on measurements of resources that are known to be repeated. However, a problem with this technique is that it cannot be used for LTE as specified in 3GPP Release 8 and below UEs, since they are not aware of the new reference signals that must be transmitted from the repeater (e.g. repeater 110-1). Hence, for support of legacy UEs in an evolved LTE system (e.g. LTE Release 10, also known as LTE-Advanced), this technique may not be used.
With respect to another technique, as illustrated in FIG. 3, eNodeB 115 may transmit a reference signal to UE 105-2. UE 105-2 may perform channel quality indicator (CQI) measurements based on the received reference signal. The measurements of UE 105-2 may be frequency selective. UE 105-2 may transmit a feedback report to eNodeB 115 via repeater 110-1. Assuming that eNodeB 115 knows the frequency selective gain vector of each controlled repeater (e.g., repeater 110-1), it is possible to derive the UE-to-repeater association from the feedback report. That is, the reference signal is transmitted by eNodeB 115, UE 105-2 performs measurements, and then provides a feedback report back to eNodeB 115. However, a problem with this technique is that it is indirect and slow. It might be required to determine the UE-to-repeater association by first turning a repeater off and then wait for CQI feedback. After that, the same repeater is turned on and the new CQI feedback can be compared with the previously received feedback. If there is any significant difference between the CQI reports received by a UE when a specific repeater is turned off or on respectively then this is an indication that the UE probably should be associated with the corresponding repeater. If there is no significant difference then eNodeB 115 might try to test the same procedure with another repeater. This procedure takes some time and also there is always the possibility that an observed difference in received CQI reports from a UE depends on something else than the fact that a certain repeater was turned on. The UE could e.g. have been shadowed by a building when the first CQI report was derived and then moved around a corner which resulted in a line-of-sight situation when the second CQI report was calculated.
In order to make efficient use of controllable frequency selective repeaters, eNodeB 115 needs to be able to determine which resources should be turned on or turned off for each controlled repeater. When assigning an uplink grant to a UE served by a repeater, eNodeB 115 ensures that the serving repeater has turned on those resources. Also, when communicating in the downlink to a UE, which is served by a repeater, eNodeB 115 uses only resources that are turned on in the serving repeater. This is typically solved by determining an association between the UE and the repeater.